Designing Built-in Test (BIT) or Built-In Self Test (BIST) capabilities into semiconductor devices (i.e. chips) is a well-known technique used to place the circuitry needed to perform critical performance tests directly on the chip rather than off chip. Without the built in test circuitry, external test equipment must be used to perform the various performance tests. Several limitations of using external test equipment include making connections to the on-chip circuitry through a limited number of contact points or pins, requiring expensive test equipment capable of performing RF measurements, and requiring an extended testing time typical of such measurements. On-chip digital or software based mechanisms, which serve to substitute for such external setups, offer significant reduction in testing costs without resulting in a noticeable increase in chip cost.
Manufacturers designing and building products such as communication devices, often design their products to operate in accordance with industry standards. One such standard is the Bluetooth short-range wireless standard. In order to ensure conformance with the standard and the design specifications for a device, qualification tests are performed on the products as part of the manufacturing process. Typically, the testing of communication devices requires the device to be connected to one or more pieces of external test equipment that perform a battery of tests to ensure compliance with the specifications. The test equipment used is typically bulky in size, costly and requires routine maintenance and calibration of its own.
In the case of a digital transmitter, the standard to which it is targeted typically specifies requirements for the modulation of the data and generation of the output signal. The Bluetooth 1.1 specification describes a frequency modulation scheme having a hopping sequence of 79 channels in the 2.4 GHz ISM frequency range. A problem arises in that modulation inaccuracies and phase noise on the oscillator that generate the RF carrier translates, after frequency demodulation at the receivers, to distortion and additive baseband noise, which could degrade the receiver's performance. Therefore, transmitters must be tested for compliance against the defined modulation quality criteria, which specifies the limits on the amount of distortion and noise in their circuitry.
A diagram illustrating the frequency deviation of an example modulated signal in accordance with the Bluetooth specification is shown in FIG. 1. The nominal frequency deviation in Bluetooth is +160 kHz for a ‘1’ and −160 kHz for a ‘0’. This nominal frequency deviation is not observed, however, for a ‘1010’ sequence where the peak deviation is reduced to 0.88·160 kHz=141 kHz, as shown in the example signal 50. This reduction in the peak deviation is the result of the intentional Intersymbol Interference (ISI) caused by the Gaussian filtering of the signal which is used to narrow the spectrum of the output transmit signal. The Bluetooth specification allows the instantaneous value of the peak frequency deviation to be further reduced to below 115 kHz only once for every 1000 bits (i.e. 0.1% error). This further reduction of frequency deviation may be caused as a result of modulation inaccuracies and distortion as well as additive modulation noise (i.e. RF phase noise). The instantaneous reduction of the frequency deviation below 115 kHz constitutes a failure event as indicated by the dotted circle 52. The width of a Bluetooth symbol is 1 microsecond corresponding to a symbol rate of 1 Mbps.
In addition, frequency droop is permitted to a certain extent by the Bluetooth specification. The amount of frequency droop permitted is 25 kHz for short packets (single time slot of 625 μsec) and 40 kHz for long packets (3-5 slots). It is noted that some open loop modulators operate by remaining closed loop until data is to be transmitted at which time they open the loop and modulate the oscillator with the packet's data.
In light of the requirements of the Bluetooth specification described above, the modulation test used by the qualification test equipment is defined such that the ‘average (or instantaneous) center frequency is determined by averaging over eight symbols (i.e.8 μsec) preceding the tested sample. The samples are taken at the centers of the symbols where frequency deviation is maximal. The tested sample is then required to be within at least 115 kHz frequency distance from that instantaneous center frequency. Only one per 1000 samples is permitted to fail this criterion. The result of this test is either a pass or fail indication.
The frequency deviation and frequency droop related requirements of the Bluetooth specification must be met by devices claiming to confirm to the standard. One approach to testing products and devices for conformance with the standard is to use specialized test equipment designed just for this dedicated purpose. In the case of a semiconductor device or chip, an alternative is to use a built-in self-test mechanism to reduce the costs associated with testing.
Using a built-in self-test mechanism to estimate the modulation noise would reduce the cost of production of the chip embodying the mechanism due to the reduction in the complexity of the tester as well as a reduction in the test time. Using today's manufacturing processes, the testing of chips may comprise a significant portion of the cost of manufacturing. Since the Bluetooth qualification tests require specific parameters and qualities for the modulated signal, it is desirable to be able to verify compliance with the standard by means of internal test hardware having minimal gate count and that requires minimal measurement time.